Although a relationship between influenza infection and pneumonia has been known for centuries, studies of populations looking at patterns of influenza and pneumococcal pneumonia have shown only a modest association at best.
Recently, a team of researchers from the University of Michigan used a novel approach, creating a computer model of transmission based on data from 1989 through 2009. The results, published in the June 2013 Science Translational Medicine journal, revealed a short-lived but significant (about 100-fold) increase in the risk of pneumococcal pneumonia following influenza infection.
Based on these data, the researchers were able to rank the likelihood of the hypotheses for why such a relationship exists. The clear winner was the susceptibility impact hypothesis, in which individuals with influenza infection have a heightened susceptibility to pneumococcal pneumonia, lasting for up to a week after the viral infection. The team from University of Michigan found that during peak flu season, interaction with influenza virus accounted for up to 40% of cases of pneumococcal pneumonia.
Given that pneumonia is one of the most deadly infectious diseases and accounts for over 1 million hospitalizations annually in the U.S., efforts to reduce the number of cases would be of tremendous health benefit. In the U.S., the most common bacterial cause for pneumonia is Streptococcus pneumonia, the cause of pneumococcal pneumonia.
The best way to reduce the incidence of bacterial pneumonia is for appropriate vaccinations for influenza and pneumococcus.